The cost of drilling oil and gas wells is extremely expensive. For example, many drilling rigs operating in the Gulf of Mexico command approximately $50,000 per day. Some drilling rigs operating in remote regions or some offshore areas cost up to $750,000 per day. Consequently, it is desirable to improve drilling efficiency by decreasing the amount of rig time required to reach the objective depth of the well. Optimizing drill bit selection type and timing can dramatically reduce the required drilling time. Knowledge of the rock properties is useful in making such drill bit selection.
Previous attempts have been made to assess formation drillability from petrophysical log measurements. Gstalder and Raynal published a paper in 1966 entitled "Measurement of Some Mechanical Properties of Rocks and Their Relationship to Rock Drillability", Jour. Petrol. Technology; August 1966; pp. 991-996, which discloses that rock hardness increases as compressional wave (P-Wave) velocities from conventional acoustic wireline tools increase. Therefore, rock hardness determined in the laboratory or estimated from acoustic logging could be used to predict drilling performance. However, no technique for predicting drill bit performance based on determined rock strength properties was disclosed. Somerton, in 1970, in a publication entitled "Well Logs Predict Drillability, Aid Computers", Oil & Gas Journal; Nov. 23, 1970; pp. 78-86, disclosed that acoustic log travel times correlated well with rock drillability, provided that a mineralogical factor is taken into account. However, no technique for predicting drill bit performance based a determined rock strength properties was disclosed.
Mason, in 1984, in a publication entitled "Three-Cone Bit Selection With Sonic Logs", SPE Drilling Jour.; June, 1987; pp. 135-142, disclosed that formation compressive strengths increase as calculated formation shear wave (S-wave) velocities increase. A correlation was made between conventional roller cone bit economic performance from various offset wells and calculated S-wave travel times. Mason's method for calculating formation S-wave velocities is extremely dependent upon mineral composition and an idealized table of P-wave/S-wave velocity ratios. Applicant's invention utilizes a more complicated and unique analytical technique to determine S-wave velocities. Also, the present invention uses electrical log derived compressive strength, cohesive strength, and angle of internal friction for diamond bit selection, as opposed to roller cone or rock bit selection. More importantly, the present invention relies on calculated rock strengths and abrasiveness estimates by bit company laboratory estimates, in addition to field performance trials, instead of the prior method of mainly comparing offset bit performance.
Onyia, in 1988, in SPE Paper No. 18166, entitled "Relationship Between Formation Strength, Drilling Strength, and Electric Log Properties", estimated rock strength from wireline logs. Laboratory measured rock core strengths were correlated to wireline acoustic and resistivity data. The core/log derived strengths were then related to the drilling performance of roller cone bits, at a close by test well. Onyia stated that given a formation rock strength and drilling parameters, approximate drilling rates can be predicted. There was no mention of selecting bits according to wireline log derived rock strengths.
Deinbach, in a Jun. 14, 1982 Oil and Gas Journal article entitled "Sonic-Gamma Ray Log Analysis Improves Bit Selection", discloses a method of selecting drill bits using sonic and gamma ray logs from nearby wells. The response of the sonic log to porosity has a close relationship to rock strength. When shale content is included, via the gamma ray log, bit selection is performed. No actual rock mechanics properties are calculated, however, and the relationships are merely qualitative.
U.S. Pat. Nos. 3,896,668 and 3,946,604, issued to Anderson, and Anderson et al. respectively, disclose methods of using sonic and density log data to determine a shaliness index, which is used to compute Poisson's ratio. Elastic constants, such as shear modulus and bulk compressibility are determined, which provide indications of a formation's strength, to ascertain competent from incompetent formations, and to help plan sand controlling techniques such as gravel packing.
None of the prior efforts described above provide a method for quantitatively determining rock mechanic's properties such as compressive strength, cohesive shear strength, and the angle of internal friction of an offset well from electrical log data, for selecting a drill bit. There is therefore a need for an improved, accurate, simple, and economic method for determining these rock properties and selecting a drill bit.